synchronous motors - Can i have this in English please

[Simon]

See attached page 9, it makes no sense, if the slip rings are short circuited how can the exciter put any voltage into the rotor? I assume that the rotor is fed with 3 phase power of opposite polarity to the stator thus mimicking the conditions of a DC fed rotor?

With this method of starting, the field of the exciter is left de-energised and the
motor is started as a normal wound-rotor induction motor. By selecting the
correct value of starting resistors, the motor can now be made to produce
maximum torque on starting and can therefore start under load.
Once the maximum speed of the induction motor has been reached, the slip
rings are short-circuited and the field of the exciter energised. Once more, a
low frequency alternating torque is superimposed on the induction motor
torque and the machine pulls into synchronism and runs as a synchronous
motor.

I think perhaps they mean the short circuit on the slip rings that was never there because they are using starter resistors in series with the rotor is removed and the energiser energises the rotor. Clearly someone was up late writing this having just finished the garbled mess that is the induction motor section of the course.

[Benta]

I understand it fully. The induction motor part is clear, and when you're close to synchronism the rotor windings are short circuited to disable the induction part.
At the same time, the starter resistors change purpose and are used to control the exciter. The schematic is a bit convolved, but hey.

[ArthurDent]

I have an old single phase repulsion start/induction motor and they are interesting. The switching mechanism is kind of complex but it works fine. Here is a video of a very old version of the motor type with a mechanical switch to go from start to run that pretty well explains the way they work.

[Simon]

I understand it fully. The induction motor part is clear, and when you're close to synchronism the rotor windings are short circuited to disable the induction part.
At the same time, the starter resistors change purpose and are used to control the exciter. The schematic is a bit convolved, but hey.

No! if you short the rotor winding's by shorting the slip rings you produce an induction motor which is what is use to start the thing not run it. The resistors would be brought to 0 as the motor speeds up making it an approximate squirrel cage motor. at this point the exciter would need to take over. I can only assume they mean short 2 of the slip rings so as to use the entire would rotor coils in the excitation.

[Benta]

Actually, it gives you a squirrel cage motor. But it doesn't matter. In synchronism there is no slip, and the squirrel cage is completely out of the picture.

[Simon]

I have an old single phase repulsion start/induction motor and they are interesting. The switching mechanism is kind of complex but it works fine. Here is a video of a very old version of the motor type with a mechanical switch to go from start to run that pretty well explains the way they work.

Clever but rather different. That is starting as a brushed motor (universal) and then being switched to induction. All to get around the not being able to start a single phase induction motor unless it is already running.

[Simon]

Actually, it gives you a squirrel cage motor. But it doesn't matter. In synchronism there is no slip, and the squirrel cage is completely out of the picture.

Well that is what i said but they are attempting to describe a motor that uses the same winding's as the induction start and synchronous rotor coils. The using separate winding's/cages is old hat by this section.

[Benta]

Actually, it gives you a squirrel cage motor. But it doesn't matter. In synchronism there is no slip, and the squirrel cage is completely out of the picture.

Well that is what i said but they are attempting to describe a motor that uses the same winding's as the induction start and synchronous rotor coils. The using separate winding's/cages is old hat by this section.

No. There are two rotor winding systems. One is the induction part, the second is the exciter. Close to synchronous operation the induction rotor coils are switched to squirrel cage operation. At the same time, the exciter is switched in and pulls the motor to synchronous revs. The squirrel cage rotor part is then no longer operative or needed.

[ArthurDent]

Simon - "That is starting as a brushed motor (universal) and then being switched to induction."

Not exactly. The only power is connected to the stator and there is no input power connection to the rotor. The opposing brushes on the rotor are shorted and become magnets from the induced power from the stator.

In a universal motor the stator and rotor windings are in series and both powered from the supply.

[Benta]

Wonderful! Now we have two subjects in one thread.
Talk about thread pollution (or riding a hobby horse).

[ArthurDent]

Although all the posts seem to be related to methods to switch from one type of motor with high starting torque to another type once running, here is another explanation using 3-phase power that is very close to the original post that might be clearer.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-13/wound-rotor-induction-motors/

[Simon]

Actually, it gives you a squirrel cage motor. But it doesn't matter. In synchronism there is no slip, and the squirrel cage is completely out of the picture.

Well that is what i said but they are attempting to describe a motor that uses the same winding's as the induction start and synchronous rotor coils. The using separate winding's/cages is old hat by this section.

No. There are two rotor winding systems. One is the induction part, the second is the exciter. Close to synchronous operation the induction rotor coils are switched to squirrel cage operation. At the same time, the exciter is switched in and pulls the motor to synchronous revs. The squirrel cage rotor part is then no longer operative or needed.

No, read it again. Remember these people are not exactly the masters of clarity. I think the short circuiting is referred to two of the slip rings converting the 3 phase rotor winding into one (virtual) DC winding. No there are not 2 winding's in this case of the schematic. Half way through the last section they stop talking about the dual type and explain how a wound rotor can be used as both wound induction winding's and the DC synchronous rotor winding's.

[Benta]

You are right, indeed only two windings are shorted together. I found a better article where the switching part is clearer:
https://electricallive.com/2015/03/synchronous-induction-motor.html

[ahbushnell]

Wonderful! Now we have two subjects in one thread.
Talk about thread pollution (or riding a hobby horse).

They should add a fork function to this forum. 

[Simon]

There is a split, if by fork you mean multiple threads in threads please NO!

[ahbushnell]

There is a split, if by fork you mean multiple threads in threads please NO!

That's a poor joke. 

[Simon]

I was not aware I was making a joke.

[BigMark]

See attached page 9, it makes no sense, if the slip rings are short circuited how can the exciter put any voltage into the rotor? I assume that the rotor is fed with 3 phase power of opposite polarity to the stator thus mimicking the conditions of a DC fed rotor?

With this method of starting, the field of the exciter is left de-energised and the
motor is started as a normal wound-rotor induction motor. By selecting the
correct value of starting resistors, the motor can now be made to produce
maximum torque on starting and can therefore start under load.
Once the maximum speed of the induction motor has been reached, the slip
rings are short-circuited and the field of the exciter energised. Once more, a
low frequency alternating torque is superimposed on the induction motor
torque and the machine pulls into synchronism and runs as a synchronous
motor.

I think perhaps they mean the short circuit on the slip rings that was never there because they are using starter resistors in series with the rotor is removed and the energiser energises the rotor. Clearly someone was up late writing this having just finished the garbled mess that is the induction motor section of the course.

Been a while since I studied this subject. Might be time to find my old Brook Crompton motor handbook.

From memory and reading some of your text.

The variable resistors are in a star configuration at one end and attached to the slip rings at the other. On start of the motor the setting on each variable resistor is high and when the motor reaches speed and the resistance setting on each resistor has dropped and eventually reaches zero ohms or therefore no longer has any effect on the motor.

As resistance has dropped to zero the resistors can be taken out of circuit and bypassed by being shorted out at the star point or at the Slip rings using a contactor. The diagram looks to be missing the contactor stage. So when the motor reaches speed the exciter is connected to two slip rings on one side and one slip ring on the other bypassing the resistor starting circuit.

[Simon]

correct, they forgot to be a bit more specific about shorting 2 rings only so that an open rotor circuit is still available to be exited. The text was identical to previous material about the starting of induction only motors and left me wondering if someone was getting carried away.

[Simon]

So a new bit of confusing syntax. See picture. Is it me or are there 2 definitions of P there? it starts as volts x amps but then becomes volts x amps divided R (Xs).

[Simon]

I am also confused at to where E has come from. I was initially told that there was no back e.m.f as there was no relative motion between the stator field and the rotor.

[ahbushnell]

I was not aware I was making a joke.

I was making a joke that was poor. 

[BigMark]

I am also confused at to where E has come from. I was initially told that there was no back e.m.f as there was no relative motion between the stator field and the rotor.

E is the vector angle on the Figure 1 diagram. All the vectors are using capital letters.

[Simon]

Yea I am not totally blind. E is "back e.m.f.". They say this having stated that synchronous motors do not have back e.m.f. I have now read alternative material and it has been made clearer that as the load angle increases because of the mechanical resistance from the load the shaft shifts from fully locked in and once the fields are not perfectly aligned back e.m.f. will be induced.